Method and arrangement for amplifying photoelectric currents

ABSTRACT

287,844. Telefunken Ges. f³r Drahtlose Telegraphie. March 26, 1927, [Convention date]. Copying-telegraphs.-In a copying-telegraph or the like comprising one or more light-sensitive devices, such as a photo-electric cell 5, one or more additional compensating light-sensitive devices 9 are arranged so that the effective output is substantially zero during light shades of a picture or the like 4 and increases with increasing darkness of shade in the picture. The invention facilitates the direct reproduction of a positive by an electro-chemical or photographic receiver. The photo-electric cells are connected in a bridge circuit which is balanced for light shades by adjusting the light falling on the compensating cell by moving a mirror 6 which reflects light from the beam falling on the cell 5, or by an iris diaphragm, or by an absorption wedge. The carrier frequency is produced bv a rapidly rotating perforated disc 7, or by a. source of alternating potential in series with a source of direct potential 10 (Fig. 2. not shown).

Jul 15, 1930, F, m R 1,710,497

' IBTHOD AND ARRANGEMENT FOR AMPLIFYING PBOTOELECTRIC CURRENTS Original Filed march 31 1928 INVENTQR ram SCHROTER J24 mm;

A I NEY' Patented July 15, 1930 UNITED-STATES PATENT OFFICE FRITZ 'soIIRo'rE oE BERLIN, GERMANY, AssIGNoR TO GESELLSGHAFT run DRAHTLOSE TELEGRAPHIE m. B. n. HALLESCHES, or BERLIN, GERMANY, A CORPORATION OF GERMANY I METHOD AND ARRANGEMENT FOR AMPLIFYING PHOTOELECTRIG CURRENTS Application filed March 31, 1928, Serial No. 266,153, and in Germany March 26, 1927. Renewed June 4, 1929.

An application for this invention has been filed in Germany, March 26, 1927, under Serial No. 33,270.

' In the telegraphic transmission of piclate a carrier wave.

tures it is a good plan to accomplish the amplification of the currents produced in the photoelectric cell, which may be of any de sired type or even a selenium cell, by the exploration of the picture in such a manner as to cause the said currents produced to modu- The carrier is then further amplified and may be sent over the transmission line for wire work or may be arranged to control a wireless transmitter.

In practicing this scheme, several methods are practicable, but among these methods only the three most important ones shall be here named.

(1) After sufiicient preliminary amplification, the direct current potentialvariations' furnished from the photoelectric cell are brought to act together with a constant alternating. current amplitude of suitable frequency, upon the gridof a socalled compound tube or thermionic tube which acts as a modulator; I

(2) The carrier frequency is obtained by applying the alternating current potential (mostly conjointly with a constant direct current potential) directly as the suction or anode potential of the photoelectric cell which is exposed to the light variations incidental to exploration of the picture and (3). The carrier frequency results through purely optical means by the rapid interruption ofth-e picture exploring light, for example, by means of a rapidly revolving disk with marginal holes. v

Of the above methods (2) and (3) offer the advantage that the complete controlor modulation of the carriage .Wave, i. e., the change from zero to maximum of the amplitude in 7 accordance with the limiting values of dark and light, or black and white in the picture,=

is constant always, even withoutstrictly accurate electrical adjustments, so that in this way high working safety is insurable. These methods, on the other hand, have so far shown this drawback that they fail to permit direct positive recording of the incoming picture readily and in all cases. Since the alternating current potential applied to the wire line or used to modulate the wireless transmitter is present only when exploring or scanning bright or light spots in thepicture, in the presence of which there is admission of reflected or penetrating light to the photoelectric cell, while for dark spots the alternating current potential drops to zero value. In the arrangements known in the prior art the au dio frequenc amplifier in the receiver ap paratus has een able to furnish current to the picture writing or recording device only as long. as the light spots in the outgoing picture came to, act upon the photoelectric cell. For instance, when using a picture recorder operating electrochemiof this kind, positive recording was directly desired, it was necessary to rectify the alternating current potential at the output end of the amplifier used to reinforce the photoelectrio current, and to bring the ensuing direct current potential which is present only in the presence of light upon a separate tone modulating device of the transmitter with an incidental blockin or stopping effect, with the result that an io frequency energy, i. e., recording current, could arise at the receiver end only in the presence of darkness. To be sure, in the case of method (1), according to the sign of the addition of pulsating photoelectric direct current potential and biasing potential at the grid ofthe thermionic 01' compound tube, it is possible to efi'ect recordfing at the receiving end either directlyin the 'form of a positive reproduction or else as a negative. However, this method! involves the drawback that the electric adjustment is very delicate, and variations in the sensitiveness of the photoelectric cell, the amplitude.

of the alternating current potential, the heating and grid biasing potentials, etc., while a transmission is proceeding, cause serious variations in the conditions of complete control or modulation of the carrier frequency. This conduces to difiiculties at the receiving end, while at the transmitter end proper manipulation of the modulation means is made diflicult.

The present invention discloses ways and means whereby direct positive recording at the receiver is made feasible also in the case of the two methods numbered (2) and (3), especially with the latter, even with such means as have before been mentioned, and without an additional reversin device between the photoelectric cell and the transmitter, comprising the rectifier tube in conjunction with another tone modulator of the radio or wire transmitter and controlled in the proper sense by the rectified potential. As a matter of fact, the purpose of such a reversing device according to the present invention, is attainable in a simpler manner by using the carrier frequency, produced by means ofa disk with marginal or peripheral perforations or suction alternating current potential at the photoelectric cell, itself for the modulation of thetransmitter.

The invention consists in that the action of the light upon the exploring photoelectric cell 'ation of a second is compensated by the irra hotoelectric cell with a correspondingly adusted luminous flux of constant, efi'ective value. This luminous flux, for exam 1e, may

\ be shunted off from the exploring lig t beam flow of the carrier frequency 1 prior to its striking the icture. As a result, n the presence of a brig t spot, and b virtue of balance effected, for instance, in t e form of a bridge arrangement, no alternating current modulating the transmitter will arise, whereas in the presence of dark, the compensation is obviated for the reason that the exploring photoelectric cell is then more or less without-current, whereas the compensating cell is further illuminated and therefore remains conducting. Hence, audio frequency modulation of the transmitter oscillation or over thetransmission line occurs only w en changing to dark in the outgoing picture, and the receiver blackening upon the supply of current records directly a ositive image of the outgoing picture (5 ackening being broadly understood as the production of a dark picture tone).

Figs. 1 and 2 illustrate by wa of example embodiments incor orating the asic idea of the present invention, though the scheme is useful not only for picture transmission, but also forother purposes in which light variations are to be recorded b photoelectric cells, and where for the contro of the indicator or receiver similar conditions are present as with a: picture recorder giving a positive image the rays from-lens'3, 5 and 9 represent the exploring and compensating photoelectric cells, respectively, which are preferably placed to form a bridge arrangement, 6 i'sa mirror or reflector partly shifted into the path of the re s from lamp 1 for shunting or diverting a efinite quantity of light upon another deflection mirror 8 so arranged as to direct the diverted light upon the photoelectric cell 9, .10 is the direct current potential source for the hotoelectric current, 11 and 12 represent additional variable direct current potential sources for compensating differences in sensitiveness or characteristics of the photoelectric cells 5 and 9 13 and 14; are the resistance arms of the bridge arrangement, 15'and 16 are diagonal junction oints which are brought to the grid and the cathode, respectively, of the amplifier tube 17, and 18, finally,

assage is a variable source of a negative grid bias-- 1 in tential for the am lifier 17.

if? p he picture is expipred by light reflected from the ori "ml to sent out rather than by assing 'ght, the arrangement, as regar s the path of the rays, has to be imagined as being changed correspondingly. In the arrangement shown in Fig. 1, the carrier frequency is produced by insertin in the trajectory of the rays the periphera row or circle of oles of a rapidly rotatin disk 7 which is arranged to interrupt the lig t from source'l at the desired frequency. In the case of 2, on the other hand, the carrier fretqluency is obtained by providing'in series wi the direct current potential source 10 an alternat current potential source 19 of any s.ui,t-' ab e frequency and value. As to the remainmg part of the structure, both arrangements are identical in o ration. To' control the quantity of the H t im ingmg' upon the cell 9, instead of shifting the mirror 6 into the ray beam, another arrangement, for instance, an iris diaphragm, a y wedge cone or 'the like may be used. at is essential is that the efiective quantiy of light which is caused to impinge upon the cell 9 is so balanced-that -in the presence of'the brightest shade occurring in the picture 4, no alternating current gtential or only a very small one, will tween points 15 and 16 of the bridge arrangement, and whatever remains can be rendered harmless by suitable-stopper means. Apart fromhavmg-recourse'to variations of direct current poteutigls between 11 and 12,

recourse may also be had to varying the resistance conditions in the other arms of the bridge. If, then, owing to a dark spot being touched in the picture, the illumination acting on the cell 5 is momentarily stopped more or less, the balance of the bridge arrangement is disturbed, and the alternating current po: tential then set up between the points 15 and 16 on the bridge acts upon the grid circuit of the tube 17 which thus furnishes an amplified alternating current potential in the plate circuit. It is this potential that then is put to further use in a way as pointed out in the beginning of the specification;

Having now described my invention,-what I claim is: I

1. In a system for amplifying varying electric current impulses produced by changes in light intensity, a plurality of photo cells, a record to be transmitted, a light source, means for influencing one of said cells by light from said source varied in intensity in accordance with the variations in light and shade in said record, means for influencing the other of said cells directly from said light source, whereby said photo cells are subjected to equal light intensities and balanced during the light portions in said rec 0rd, an amplifier associated with said photo cells, means for impressing an alternating current frequency upon said cells, means for modulating said frequency in accordance with variations in light and shade reaching the first of said photo cells and means for applying said frequency to said amplifier during periods of unequal light intensities upon each of said photo cells.

' sities upon said cordance with of said photo cells provided by an 2. In a system for amplifying varying electrical current impulses produced by changes in light intensity in a record to be transmitted, a pair of photo cells each arranged to form one leg of a bridge structure, means for subjecting one of said cells to varying light intensities in proportion to the intensities of light and shade in a record to be transmitted, means for subjecting the other to a constant illumination whereby for highlights in said record said cells are balanced, means for introducing an alternating current frequency in said bridge diagonal ofsaid bridge structure, and means unbalanced condition in said bridge structure due to unequal light intenternating current frequency modulated in acsaid light intensities to said amplifier.

3. In a system for amplifying varying electric current impulses produced by variations in light and shade in a record to be transmitted-afiecting the amtmnt of light reaching the light sensitive medium, a pair of photo cells, means for subjecting one of modulated alternating current "said cells to variations in light intensity in cells being connected bridge formation and arranged to each form one leg of said bridge structure, whereby forco-nditions of maximum light intensity reaching the first of said photo cells said bridge structure is substantially balanced, means provided in the legs of said bridge structure includhig said cells for compensating for differences in sensitiveness of said cells, means for introducing an alternating current frequency upon said bridge structure, means provided by varying light intensities reaching the first of said photo cells for modulating said alternating current frequency, an amplifier, and means provided by an unbalanced condition in said bridge structure for applying a modulate'd alternating current frequency input circuit of said amplifier.

- FRITZ SCHROTER,

'cells for applying said 31- upon the 

